148 
PI.EDRONECTID*. 
coui’se, the pectoral fin for the most part is brought into 
action for this purpose. 
In separating this numerous family into subordinate groups, 
it is convenient and usual to divide them principally accord- 
ing as when the belly is towards the observer the head and 
eyes are directed to the right or left. But this particular 
structure is not known to exert any influence on the habits 
of the individual species; and, therefore, in combination with 
this it is of importance to include in our consideration other 
material variations of form; which, therefore, are placed among 
the essentials of the definitions of the genera. 
